In the TOFMS, the time of flight required for an ion packet (an aggregate of ions) ejected from an ion source supplied with a certain level of kinetic energy to reach a detector is measured, and the mass (or mass-to-charge ratio m/z, to be exact) of each ion is calculated from the time of flight. One major cause of deterioration in the mass-resolving power is spread in the initial energy of the ions. Spread in the initial energy of the ions ejected from the ion source causes broadening in the time-of-flight of the ions of the same mass, and deteriorates the mass-resolving power. To compensate for the time-of-flight broadening due to the initial energy spread of the ions, ion reflectors have been widely used. A TOFMS using the ion reflector is hereinafter called the “reflectron” according to the common practice.
An ion reflector has an electric potential distribution in which the potential increases in the traveling direction of the ions, and has the function of reflecting ions coming through a drift space with free of electric field. An ion having a larger initial energy (initial speed) penetrates deeper into the ion reflector, and hence spends a longer time flying in the ion reflector when reflected. On the other hand, the ion having a larger initial energy flies at a higher speed and hence spends a shorter time flying through a non-electric field drift space. Therefore, by appropriately adjusting the parameters so as to cancel the increase in the time of flight in the ion reflector by the decrease in the time of flight in the non-electric field drift space, the total time of flight from the ion source to the detector becomes almost independent of the initial energy within a certain range of energy (see Non-Patent Literature 1 for details).
Various types of reflectrons have been developed. A well-known reflectron is a dual-stage reflectron which was first developed by Mamyrin et al. (see Non-Patent Literature 2). FIG. 8A is a schematic diagram showing an ion path in the dual-stage reflectron. FIG. 8B is a schematic diagram of a potential distribution on the center axis.
In the dual-stage reflectron, an ion reflector is constructed by two stages of uniform electric fields (a uniform electric field is an electric field in which the potential changes proportional to the distance), i.e., a first stage region S1 and a second stage region S2. Grid electrodes G1 and G2 including a large number of openings through which ions can pass are respectively set in the boundary between a non-electric field drift region and the first stage uniform electric field (the first stage region S1) and the boundary between the first stage uniform electric field and the second stage uniform electric field (the second stage region S2). That is, the non-electric field drift region and the first stage region S1 are partitioned by the grid electrode G1. The first stage region S1 and the second stage region S2 are partitioned by the grid electrode G2. Usually, the first stage region S1 is shorter than the second stage region S2, and, provided that approximately two thirds of the initial energy of ions is lost in the first stage region S1, the total time-of-flight spread is compensated to the second derivative of the energy (that is, the second-order energy focusing is achieved). Therefore, the time-of-flight broadening for an ion packet having initial energy spread to some extent can be small. As a result, high mass-resolving power is obtained. Such a dual-stage reflectron is most widely used in commercially available time-of-flight mass spectrometers.
As explained above, in the dual-stage reflectron, basically, the electric fields are uniform electric fields in the stages of the ion reflector. It is known that energy-focusing performance can be improved by appropriately correcting the potential distribution of a part of the electric field to be a non-uniform electric field. For example, in Patent Literature 1, the present inventors propose a new TOFMS that realizes isochronism for an ion packet having energy equal to or larger than a certain energy threshold and flying on the center axis, by slightly correcting the potential distribution of the second stage region S2 in the dual-stage reflectron.
FIG. 9 is a schematic diagram of the potential distribution in the dual-stage reflectron described in Patent Literature 1. The position P in FIG. 9 is a second-order focusing position in the conventional dual-stage reflectron in which correcting potential is not superimposed. In a deeper space starting from the second-order focusing position P, correcting potential ZC(U) proportional to {U(Z)-E0}3.5 is superimposed on potential ZA(U) of the uniform electric field. If the correcting potential ZC(U) is not superimposed, the time-of-flight spread is compensated for up to the second derivative of energy (the conventional technique of Mamyrin solution). However, the time-of-flight spread is compensated for up to the third and infinitely continuing higher-order derivatives that cannot be compensated by the Mamyrin solution, by superimposing correcting potential ZC(U). Consequently, complete isochronism can be realized for ions reflected in the correcting potential portion. Potential distribution curves are smoothly connected before and behind the second-order focusing position P. Further, the correcting potential ZC(U) is extremely small compared with the potential ZA(U) of the uniform electric field. Therefore, not only theoretically, it is relatively easy to actually superimpose such correcting potential ZC(U). In the explanation, Z represents a coordinate along the center axis of the ion reflector, U represents a potential value in the coordinate Z, and E0 represents a potential value in the second-order focusing position P.
According to the method explained above, it is possible in principle, to realize an ideal reflectron. In actual, it is necessary to form a theoretically calculated ideal correcting potential distribution on the center axis inside the ion reflector. However, it is difficult in a conventional general ion reflector to form this highly accurate potential distribution. A reason for this is explained below.
In general, an ion reflector forms an ion reflection electric field in its internal space with a plurality of guard-ring electrodes. FIG. 10 is a configuration diagram of a general ion reflector 4 including a plurality of guard-ring electrodes. A guard-ring electrode 401 is a substantially annular metal plate including an opening in the center. The shape of the opening is various, such as circular or rectangular, according to the path shapes of ions. Between adjacent guard-ring electrodes 401 of thickness Te, an insulating spacer 402 having thickness Ts is disposed. Thus, the interval between the adjacent two guard-ring electrodes 401 is Ts. As shown in the drawing, in the conventional general dual-stage reflectron, the guard-ring electrode 401 and the spacer 402 having the same shapes are used in the first stage region S1 and the second stage region S2. The main reason is to reduce costs by using the guard-ring electrode 401 and the spacer 402 in common.
The mass-resolving power of the general TOFMS currently on the market is 10000 or more. To realize the high mass-resolving power to this extent, it is necessary to dispose the guard-ring electrode 401 at high position accuracy in micron order. Therefore, it is necessary to manufacture the guard-ring electrode 401 and the spacer 402 at high accuracy, and further assemble them at high accuracy. Patent Literature 2 describes a method of disposing guard-ring electrodes at high position accuracy and inexpensively realizing the guard-ring electrodes. In the literature, the thicknesses of a plurality of guard-ring electrodes are the same, and the interval between adjacent electrodes, that is, the thicknesses of spacers, are also the same.
To form the non-uniform ideal potential distribution as described above along the center axis inside the ion reflector, it is desirable to dispose as many number of guard-ring electrodes at as narrow intervals as possible (i.e., at as high density as possible). It is also desirable to make the guard-ring electrodes as thin as possible. Further it is desirable to make the inner circumferential edge of the guard-ring electrodes as close as possible to the center axis.
The above explanation about the disposition and the shape of the guard-ring electrodes is illustrated using an example of simulated calculation on potential distributions in the inner space of the guard-ring electrodes. The configuration and the shape of the guard-ring electrodes used for the calculation are shown in FIG. 11A. The guard-ring electrodes have a shape rotationally symmetrical with respect to the Z axis. The diameter of the opening through which ions pass is 100 [mm]. Both the thickness Te of the guard-ring electrodes and the thickness Ts of the spacers (the interval between the adjacent electrodes) are 10 [mm]. The grid electrode G is placed at a position half the thickness of a guard-ring electrode, that is, the position of Tf=Te/2=5 [mm] thickness. To form a uniform electric field along the Z axis in the guard-ring electrodes having such a shape, applied voltages to the guard-ring electrodes are set to 0, 200, 400, 600, 800, and 1000 [V] respectively from the incident end electrode.
FIG. 11B shows a calculation result of the potential distributions formed in the spaces in the guard-ring electrodes. Equipotential surfaces are shown at a 20 [V] interval. FIG. 12 is potential distributions on the Z axis (Y=0) and the line parallel to the Z axis at Y=50 [mm]. FIG. 13 is an ideal potential distribution of the uniform electric field (Videal) and a distribution of deviation (ΔV=V−Videal) between the ideal potential of the uniform electric field and potential actually formed on the Z axis and lines parallel to the Z axis at Y=10, 20, 30, 40, and 50 [mm].
The following is found from the results shown in FIG. 11 to FIG. 13.
(1) According to FIG. 12 and FIG. 13, although the actual potential distribution is close to the ideal potential of the uniform electric field near the center axis (Y=0) of the ion reflector, the deviation between the ideal potential and the actual potential increases at a position further away from the center axis and closer the guard-ring electrode 401 (i.e., Y is larger).
(2) As shown in FIG. 11B, a curve of an equipotential surface is larger at a position closer to the guard-ring electrode 401. Since it is certain that, if the guard-ring electrode 401 is thinner, the curvature will be smaller (curve will be milder), it is apparent that the deviation of the potential explained in (1) is caused by the thickness of the guard-ring electrode 401. In other words, it is considered that, as the guard-ring electrode 401 is thinner, deviation of the potential at a position away from the center axis by distance Y is smaller (the deviation is zero if the guard-ring electrode is infinitely thin).
As explained above, the guard-ring electrode should be as thin as possible to form the ideal potential distribution in the ion reflector. However, actually, there are limitations. As shown in FIG. 8B and FIG. 9, the grid electrodes G1 and G2 are provided respectively at the boundary between the non-electric field drift region and the first stage region S1 of the ion reflector, and at the boundary between the first stage region S1 and the second stage region S2 of the ion reflector in order to form electric fields having different strengths on the both sides of the boundaries and to allow ions to pass. If the grid electrode G1 or G2 has bent or slack, distortion in the potential distribution inside the ion reflector appears. Therefore, to achieve high performance, it is necessary to stretch the grid electrodes at high flatness. For example, Non-Patent Literature 3 describes a method of stretching the grid electrodes without slack. If the grid electrodes are stretched on the inner circumferential wall surface facing the center opening of the guard-ring electrode, structurally speaking, the guard-ring electrode needs to be thicker than a certain value. Typically, to stretch the grid-electrodes without slack, the thickness of the guard-ring electrode needs to be approximately 5 to 10 [mm] or more.
In a so-called gird-less reflector, commercialized by some manufacturers, which does not use a grid at the boundary before and behind a first stage region, in some case, the thickness of a guard-ring electrode is as thin as approximately 2 [mm] or less. However, it is practically impossible to stretch a grid electrode at such thickness of a guard-ring electrode. In such a grid-less reflector, as in the ion reflector using grids, the guard-ring electrode and the spacer having the same shapes are respectively used in all regions in common.
In the simulation, the thickness of the guard-ring electrode is set to 10 [mm] taking into account such circumstances. However, as it is evident from the above result, when the guard-ring electrode is thick to this degree, unevenness of a potential distribution at a position, in particular, away from the center axis in the radial direction is conspicuous. As a result, even if it is attempted, for example, to superimpose the correcting potential on the potential of the uniform electric field to form the ideal potential distribution, the deviation between the actually obtained potential and the ideal potential increases and deterioration in isochronism for the ion packet increases.
In the following explanation, with respect to the guard-ring electrodes of the ion reflector, such terms as “thick electrode” and “thin electrode” are used. In relation to the conventional technique explained above, the “thick electrode” indicates an electrode having thickness of approximately 5 to 10 mm or more. On the other hand, the “thin electrode” indicates an electrode having thickness of approximately 2 [mm] or less.